The cytolethal distending toxins are a family of heat-labile protein cytotoxins produced by several different bacterial species including diarrheal disease-causing enteropathogens such as some Escherichia coli isolates, Campylobacter jejuni, Shigella species, Haemophilus ducreyi and Actinobacillus actinomycetemcomitans. There is clear evidence that Cdts are encoded by three genes, designated CdtA, CdtB, and CdtC which are arranged as an apparent operon. These three genes specify three polypeptides designated CdtA, CdtB and CdtC with apparent molecular masses of 28, 32 and 20 kDa, respectively, that form a heterotrimeric holotoxin. Several cell lines and cell types have been shown to be sensitive to Cdt; these include human lymphoid cells, fibroblasts, human embryonic intestinal epithelial cells, a human colon carcinoma cell line, and human keratinocytes, among others. In response to Cdt, most of these cells exhibit G2 arrest, cellular distension and eventually cell death. However, the effects of Cdt on lymphocytes are different; Cdt-treated lymphocytes do not exhibit cellular distension and are nearly five orders of magnitude more sensitive to Cdt (10-50 pg/ml) relative to most other cells (1-5 μg/ml).
There is compelling evidence that CdtB must be internalized to induce cell cycle arrest. Several investigators have also suggested that CdtB functions as a DNase-like moiety whereby it cleaves DNA and activates the G2 cell cycle checkpoint. This mechanism of action, however, does not account for the huge difference in lymphocyte sensitivity to the toxin. It has been shown that Cdt-treated cells exhibit DNA degradation. However, it has also been shown that Cdt-induced DNA fragmentation in lymphocytes is not the result of direct effects of the toxin, but rather the irreversible effects of cell cycle arrest leading to activation of an apoptotic cascade. The bias in the art toward rationalizing CdtB function is mostly based on its homology with DNase. This bias has obscured the fact that its protein fold, and most likely the reaction mechanism, are also shared with many proteins found in a family of functionally unrelated signaling metalloenzymes that includes phosphatidylinositol (PI)-5-phosphatases.
Therefore, there is a clear need for developing strategies of both inhibiting and using the immunotoxin for therapeutic purposes.